1. Technical Field
The present disclosure relates to vibration damping systems and, more particularly to vibration damping systems for elongate rotating shafts.
2. Background of Related Art
A typical arrangement of shafting includes several relatively short segments, a number of grease-lubricated rolling element hanger bearings, and often mechanical couplings to allow for shaft misalignment or to produce an intended curvature in the shaft to conform to various geometric constraints. The reason that conventional shaft segments are relatively short is in order to maintain the overall critical speed of the shaft (i.e., the speed at which resonance occurs, its natural frequency) above the operating speed. Normally there is a trade-off between a relatively short and thick shaft transmitting a large torque or a relatively thin and long shaft transmitting a small torque and rotating at high speed. The present disclosure is concerned with the latter alternative because the distance is relatively large. Therefore, the shaft is rotating at a relatively high speed. Accordingly, to save weight, these drive shafts are generally in the form of hollow tubes. As such, if the operating speed approaches the critical speed of the shaft, without provisions to maintain control of the shaft as the shaft nears and passes through resonance, there is potential for damaging loads and even loss of the shaft and its power-transmitting capacity.
Stated differently, as the rotational velocity of a shaft increases, the shaft passes through several progressively higher speed ranges known as critical speeds. If a shaft is unsupported intermediate of its ends and is rotated at or near what is known as its first order critical speed, centrifugal forces periodically exceed centripetal forces at the shaft""s median portion to bow the shaft outwardly from its normal rotational axis, whereupon shaft rotation becomes unstable and excessive vibration occurs. Such excessive vibration may quickly damage bearings and bearing support structures, as well as the shaft itself, and may result in the impact of the shaft against housing elements or in the total destruction of the shaft itself.
A shaft""s first order critical speed is inversely related to its length and directly related to its rigidity which, in turn, is directly related to the shaft""s diameter. When a shaft is long relative to its diameter, it is more susceptible to whipping caused by an unbalanced mass distribution, which unbalanced mass distribution only aggravate whipping as the rotational speed increases.
Ideally, a shaft would be assembled from a tube that is absolutely round, absolutely straight, and which has uniformly thick walls. Unfortunately, this condition is never found in actual practice. In practice, the cross sections of the tubes may be distorted, the tubes may be bowed and the walls may include regions which are relatively thinner and thicker.
Prior attempts to damp shaft vibration or to otherwise modify shaft vibration modes have had only limited success. For instance, a drive shaft may be equipped with a dynamic damper consisting of a spring and mass system attached to the drive shaft which is tuned to the frequency of the vibration of the drive shaft which is desired to be controlled. The dynamic damper changes the resonance property of the drive shaft so as to suppress the oscillation level at the original resonance frequency of the drive shaft. With the aim of suppressing the resonant vibration of the drive shaft, it has been suggested that the drive shaft may be divided into two segments each of which are separately supported by the vehicle body. By thus reducing the span of the drive shaft (i.e., making the shaft shorter), the resonance frequency of each span is raised, in some cases, well above the frequency of the vibration induced by the rotation of the drive shaft. However, this arrangement of a multiplicity of shorter shafts is not always effective when transmitting rotation over long distances.
In addition, prior approaches addressing the vibrational problems of rotating shafts are described in Matheny, Jr., U.S. Pat. No. 3,897,984, issued Aug. 5, 1975 and Seibel, U.S. Pat. No. 2,652,700, issued Sep. 22, 1953.
Matheny, Jr. provides a shaft support comprising a generally annular resilient member disposed slightly eccentrically about the shaft generally centrally thereof. The resilient member has means associated therewith for exerting a radial preload force on the shaft. The force exerting means includes an annular roller bearing disposed about the shaft and contacting a sleeve thereon. Shaft vibration is thereby damped but at the expense of constant bearing contact and, therefore, constant wear of the shaft and the bearing structure, with consequent power losses due to friction.
In Seibel, a shaft extends through the central aperture of a damper plate which is mounted to the shaft housing by springs. The plate is contacted by a sleeve on the shaft during rotation thereof to absorb energy to prevent transmittal of shock to the supported structure. Structures such as in Seibel tend to be noisy, involve relatively many parts and suffer excessive wear.
Accordingly, a continuing need exists for an improved vibration damping system for use with rotating shafts which overcomes the above noted disadvantages of prior art damping systems.
Vibration damping systems for use in conjunction with rotating hollow bodies are provided. In one embodiment of the disclosure, the vibration damping system includes a tubular outer shaft having a proximal and a distal end and defining a lumen therethrough, a tubular inner shaft having a proximal and a distal end wherein the inner shaft is configured and dimensioned to be received within the lumen of the outer shaft and a plurality of toroidal vibration damping elements disposed between the proximal and the distal ends of the outer and inner shafts. Preferably, at least one vibration damping element is affixed between the proximal and the distal end of the outer and inner shaft and each end most vibration nodal point of a series of nodal points located along a length of the outer and inner shafts. In addition, preferably, at least one vibration damping element is affixed between adjacent vibration nodal points of the series of nodal points along the length of the outer and the inner shafts.
Moreover, the vibration damping elements of the vibration damping system further include an outer perimetral surface configured and dimensioned to contact an inner surface of the tubular outer shaft and an inner lumen configured and dimensioned to receive the inner tube therethrough. Preferably, each vibration damping element is affixed to the outer tube at approximately 90xc2x0 intervals, however, it is contemplated that each of the vibration damping elements can be affixed to an inner surface of the outer tube along the entire periphery thereof as well as being affixed to the outer surface of the inner tube along an entire periphery thereof.
In an alternative embodiment, each vibration damping elements of the vibration damping system includes a collar defining a longitudinal opening therethrough and an orthogonally oriented disk extending radially outward from the collar. The collar is configured and dimensioned to receive the inner tube within the opening thereof while the disk is configured and dimensioned to contact an inner surface of the tubular outer shaft. Each vibration damping element being secured in place by the collar being affixed to an outer surface of the inner tube and the edge of the disk being affixed to an inner surface of the outer tube.
In yet another embodiment, the disk includes a proximal rim formed along a periphery of the disk and a distal rim formed along the periphery of the disk. The proximal and distal rims defining a circumferential channel therearound. In this manner, the disk is affixed to the tubular outer shaft by deforming the tubular outer shaft circumferentially along the longitudinal location of the channel of each of the vibration damping elements.
In an alternative embodiment, the vibration damping system includes a tubular outer shaft and at least one vibration damping element internally affixed within the tubular outer shaft. The vibration damping element being affixed at a location between each end of the tubular outer shaft and each end most nodal point of a series of vibration nodal points of said tubular outer shaft. Vibration damping elements also being affixed between each adjacent nodal point of the series of vibration nodal points.
Preferably, the vibration damping element includes a first vibration damping element having an outer surface configured and dimensioned to contact an inner surface of the tubular outer shaft and an inner surface defining a lumen and a second vibration damping element configured and dimensioned to be received within the lumen of the first vibration damping element. The second vibration damping element being affixed to the inner surface of the first vibration damping element at a location between each end of the tubular outer shaft and each end most nodal point of the series of vibration nodal points. In addition, the second vibration damping element is affixed between each adjacent nodal point of the series of vibration nodal points.
In yet another embodiment of the present disclosure, the vibration damping system includes a single cylindrical vibration damping element configured and dimensioned to be received within the tubular outer shaft.
It is an object of the present disclosure to provide a vibration damping system for rotating hollow bodies which overcomes the drawbacks of prior art vibration damping systems.
It is a further object of the present disclosure to provide a vibration damping system for rotating hollow bodies which is effective in increasing the effective rigidity of a rotating object against vibration and which controls undesired vibrations of the rotating hollow body.
It is still a further object of the present disclosure to provide a vibration damping system for rotating bodies which would not substantially increase the mass of the rotating body.
These objects and advantages, together with other objects and advantages of the presently disclosed vibration damping system, along with the various features of novelty which characterize the disclosure, are pointed out with particularity in the claims annexed to and forming a part of this disclosure.